The present invention relates to radiation sensitive compositions (photosensitizers) and to positive working photoresist compo-sitions containing such photosensitizers, and particularly to photoresist compositions containing a novolak resin, such as those disclosed in copending patent application U.S. Ser. No. 07/952,451 filed on Sept. 28, 1992, together with a blend of photosensitizers, which are: a trishydroxyphenylethane 2,1,5-/2,1,4-diazonapthoquinone sulfonate, and a trihydroxybenzophenone 2,1,4-diazonaphtoquinone sulfate.
It is known to the skilled artisan to produce positive photoresist compositions, such as those described in U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. These include water insoluble, aqueous alkali-soluble phenol-formaldehyde novolak resin resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and photosensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the photosensitizer is not soluble prior to exposure. Upon imagewise exposure of portions of the coated substrate to actinic radiation, the photosensitizer is rendered alkali soluble and the exposed areas of the coating become more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in or otherwise contacted with an alkaline developing solution, while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate. The exposed and developed substrate is usually thereafter subjected to an etching process. The photoresist coating protects the coated areas of the substrate from the etchant and the etchant is only able to etch the uncoated areas of the substrate, which correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to created selective exposure patterns on the coated substrate prior to development. The relief pattern of photoresist on a substrate produced by this method is useful for various applications including the manufacture of miniaturized integrated circuits.
The characteristics of the photoresist compositions, which are important in commercial practice, include its photospeed, contrast, resolution (edge acuity), thermal stability of the image during processing, processing latitude, line width control, clean development and unexposed film loss.
Photoresist contrast is the slope of the linear portion of the curve resulting from the plot of the log of exposure energy vs. normalized film thickness remaining under fixed development conditions. In use, development of an exposed photoresist coated substrate is continued until the coating on the exposed area is substantially completely dissolved away. Resolution refers to the capability of a photoresist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths usually on the order of one micron (micrometer) or less. This capability to reproduce very small dimensions, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the photoresist. Although negative photoresists, wherein the exposed areas of photoresist coating become insoluble and the unexposed areas are dissolved away by the developer, have been extensively used for this purpose by the semiconductor industry, positive photoresists have inherently higher resolution and are utilized as replacements for the negative photoresists.
In photoresist technology, it has normally been desired to increase photoresist contrast. High contrast positive working photoresists produce developed images which exhibit high edge acuity, when exposure is performed on typical equipment such as steppers and projection aligners. In most lithographic semiconductor applications, the high edge acuity of developed images is of great importance since it allows for small variations of line width over the wafer's topography. Therefore, it permits good control of etching during anisotropic plasma-etching and is typically associated with good processing latitude.